Phylum Brachiopoda Lecture 13 PDF

Phylum Brachiopoda  Phylum Brachiopoda Brachiopods were the dominant shelly marine invertebrates of the Palaeozoic. Brachiopods originated in the early Cambrian and diversified in the Ordovician. Although they survived t...

Phylum Brachiopoda  Phylum Brachiopoda Brachiopods were the dominant shelly marine invertebrates of the Palaeozoic. Brachiopods originated in the early Cambrian and diversified in the Ordovician. Although they survived the end-Permian mass extinction they declined through the Mesozoic. They are classified into three subphyla: Linguliformea and Craniiformea, and Rhynchonelliformea. They are exclusively marine filter feeders. The brachiopod shell shape is sometimes indicative of substrate type. Communities of brachiopods can be used to study paleoenvironments. Some groups like the lingulids have changed very little in shape over the last 500 million years. This is why some brachiopods are referred to as living fossils. If they have a common name, it is “lamp shells,” since the shells of one living group, the terebratulides, bear some resemblance to the oil lamps of Biblical times. Most modern brachiopods live in marginal marine environments attached to the substrate and species tend to be morphologically similar. The “lamp shells” got their nickname from However, Palaeozoic brachiopods exploited a wider range of marine their resemblance to a Biblical oil lamp. environments and their morphology was extremely diverse, ranging from erect coral-like forms to flattened saucer shapes.  Phylum Brachiopoda Morphology The typical brachiopod shell has one valve that is larger than the other. It is called the ventral (or pedicle) valve, since it usually has an opening for a fleshy stalk called the pedicle, with which the brachiopod attaches itself to the substrate. The opposite, smaller valve is known as the dorsal (or brachial) valve, because the lophophore (brachium) attaches to it. Lophophore (filters particles and detritus out of the water) occupies most of the shell’s internal volume in the mantle cavity. Crammed into the back of the shell is the coelomic cavity, which contains most of the internal organs. There is also a kidney-like nephridium used for excretion of metabolic wastes. The brachiopod has no eyes to sense light, but a set of bristles around the margin called setae help sense the environment. The adductors run perpendicularly from the dorsal to the ventral valves, and pull the two valves together, closing the shell. The diductors insert on the middle of the ventral valve, and also on the cardinal process of the dorsal valve, so they pull the dorsal valve around its hinge line and cause it to open.  Phylum Brachiopoda Morphology The opening for the pedicle is called the pedicle foramen, and it usually perforates the beak, or the pointed part of the ventral valve. The pedicle foramen may be enclosed on the anterior end by a single plate, called a deltidium, or by a pair of deltidial plates. Pedicle foramen In some groups, there is no round pedicle foramen, but instead a notch called a delthyrium. If the delthyrium is shallow, then a similar notch on the dorsal valve called the notothyrium may enlarge the pedicle opening. If a plate encloses the notothyrium, it is called the chilidium, or there may be a pair of chilidial plates. In pentameride brachiopods, there is a large, spoon- Delthyrium shaped platform in the ventral valve for the hinge muscles called the spondylium. This group is also characterized by a median septum, and a corresponding spoon-shaped feature in the dorsal valve called the cruralium. The septum, cruralia, and spondylia of the pentamerides subdivides the internal volume of the shell into five chambers that give the group its name. Morphology Phylum Brachiopoda The calcareous support of the lophophore, or brachium, is called the brachidium, and it is attached to the hinge area by the crus (plural, crura). It may have an elaborate loop to support the main part of the lophophore. The shape of the brachidia can be highly variable. In many spirifers they are arranged in a conical spiral pointing laterally. The hinge line can be straight (strophic) or curved (astrophic). It can have a large flat or curved surface between the beak and the posterior margin of the other valve, which is known as an interarea. The convex posterior portion extremity of the shell is known as the umbo. Some hinges come to wing-like lateral points called the cardinal extremities. The edge of the shell along its line of closure is called the commissure. It may be straight, or have corrugated edges (plication), and it may also have a deep trough on one valve (sulcus) that is matched by a large elevated area (fold) on the opposite valve. The surface of the shell may show concentric growth lines, indicating the enlargement of the shell from its embryonic origin around the hinge area, as well as fine radial ribs that run from the beak to the commissure known as costae.  Phylum Brachiopoda Morphology Astrophic Strophic  Phylum Brachiopoda Classification Traditional classification: inarticulates vs. articulates Articulate 1. Inarticulates: shells lack defined hinges and are made of calcium Inarticulate phosphate (phosphatic); they do not have teeth or sockets so rely on muscles to hold and move them. Example: Order Lingulida. 2. Articlulates: shells with articulated hinges (with teeth and sockets) and made of calcium carbonate. Examples: all brachiopods other than Lingulida. Modern classification: Subphyla Linguliformea, Rhynchonelliformea, and Craniiformea Linguliformea Craniiformea Rhynchonelliformea  Phylum Brachiopoda Subphylum Linguliformea Order Lingulida (Cambrian to Recent) The lingulides are best known from the common living genus, Lingula, which lives buried in mudflats using its long pedicle to dig a burrow. Their extraordinary longevity and conservatism is probably due to the fact that they are very successful at living deep in the mud of brackish lagoons, and have tracked that common habitat through 600 million years with little pressure to change in response to predators or competitors. Lingula Subphylum Craniiformea Order Craniida (Ordovician to Recent) Craniid brachiopods are circular to subcircular in shape. Their dorsal valve is calcified and they tend to encrust onto hard surfaces including other brachiopod shells. Isocrania costata Subphylum Rhynconelliformea Phylum Brachiopoda Order Athyridida (Ordovician to Jurassic) They have a complex spiral brachidium and retain their pedicle after maturation. Order Atrypida (Ordovician to Devonian) Juvenile atrypids possess a pedicle, which is lost during maturation. Composita subtilita They were among the first brachiopods to evolve a spiral- shaped lophophore brachidium. Known to be a tropical group, they were common at the beginning of the Paleozoic, but were wiped out by the Late Devonian Frasnian-Famennian extinction. Order Orthida (Cambrian-Permian) They have a long, straight (strophic) hinge, with a wide open triangular delthyrium and notothyrium, surrounded by a distinct Atrypa spinosa but narrow interarea on both valves. The majority of the taxa also have very fine costae radiating from the cardinal area, a gently biconvex shell that is circular or elliptical in outline, and fold and sulcus that are shallow or absent. Internally, they have no brachidium, but a short brachiophore instead. Hebertella  Phylum Brachiopoda Order Pentamerida (Cambrian-Devonian) Most pentamerides had deeply biconvex shells with highly curved (astrophic) hinges, and a small uncovered delthyrium-notothyrium. Their most distinctive feature is the large, scoop-shaped spondylium in the ventral valve, and a large septum and cruralium complex, dividing the inside of the shell into five chambers (hence the name pentamerides). Although pentamerides were never as diverse as other suborders, their Pentamerus oblongus great numerical abundance in the Silurian makes them a useful index of that period. Order Productida (Ordovician to Permian) Productids are best known for their spine-covered shells. Spines functioned as protection and acted as anchors when living on soft substrates, although some also cemented themselves to harder substrates. Order Rhynchonellida (Ordovician-Recent) Pulchratia symmetrica It has remained little changed since the Ordovician. Never abundant compared to other brachiopods, they managed to persist nevertheless, and today there are a few surviving genera. They have a short, bent hinge line and a pointed pedicle beak giving their posterior profile a distinctive pointed “V” shape. Most have a pedicle, and the delthyrium is partially closed. The majority of the genera have coarse plicated costae or ribs, giving them a highly crenulated, zig-zag commissure, and a deep fold and Camarotoechia sulcus.  Phylum Brachiopoda Order Spiriferida (Ordovician-Jurassic) This highly diverse and distinctive group got its name from the spiral arrangement of its lophophore. Spiriferides tend to have highly biconvex shells, with well- developed radial costae, and a large interarea on the ventral valve. The most typical group, the suborder Spiriferidina, are characterized by a very long, strophic hinge line, giving them a wing-like profile. This group typically has a very wide pedicle interarea with a large triangular delthyrium, and a deep fold and sulcus. Mediospirifer audaculus Order Strophomenida (Ordovician to Triassic) Strophomenides are the largest and most variable of brachiopod orders, with a number of unusual forms. They are typically concavo-convex to plano-convex, with a long, straight (strophic) hinge line, and a completely closed pedicle foramen. This implies that their pedicle was reduced or absent, and not important in adults, so it required them to live freely on the substrate once they matured enough to lose their pedicle. The earliest and most primitive group is the suborder Strophomenidina, which have a distinctive shell that is deeply Strophodonta demissa concavo-convex and has a long strophic hinge, giving them a “D”-shaped outline.  Phylum Brachiopoda Order Terebratulida (Devonian-Recent) The best known of the living brachiopods are the terebratulides, which have a distinctive shape often compared to a biblical oil lamp. Shell is strongly biconvex, with a large pedicle foramen (and therefore a large pedicle) and beak that overhangs the short, curved hinge with no interarea. Since there is a large pedicle foramen, the delthyrium is closed by delthyrial plates. Most taxa have a relatively smooth shell with little ornamentation and no fold or sulcus. Internally, they have a loop-shaped brachidium. Terebratulina septentrionalis  Phylum Brachiopoda Ecology and paleoecology Brachiopods are exclusively benthic marine animals. As filter feeders they do not actively search for food and most brachiopods live on, or partially enclosed by, the substrate. They are dependent on currents to bring food and oxygen and carry away waste products. Fossil forms exploited a range of benthic habitats.  Phylum Brachiopoda Discriminating between brachiopods and bivalves Brachiopods look superficially very similar to bivalves. This similarity is the consequence of sharing a similar lifestyle; most species of each group are sessile filter feeders living in the shallow marine environment. As such they represent an example of evolutionary convergence. Both have bilateral symmetry. Bivalves grow a left and right shell with the line of symmetry along the margins of the valves. Brachiopods grow a front and back (dorsal and ventral) shell, with the line of symmetry cutting each valve in half. Most brachiopods have a plane of symmetry that runs through both shells, so that one half of the shell is the mirror image of the other. By contrast, most clams are symmetrical between the valves, so the right valve is the mirror image of the left valve. Of course, there are many groups that break this rule, such as the scallops.